Pipes made of plastics material are used extensively in industry and in construction. Many applications now use pipes made from plastics materials such as polyethylene and polypropylene rather than metal. One such application is the pipework used in petroleum installations such as garage forecourts.
In the design of petroleum forecourt installations for example, it is regarded as increasingly important to contain and detect any leaks of petroleum or diesel fuel from subterranean pipes that connect one or more storage tanks to dispensing pumps in the installation. To that end, many current designs of forecourt installation utilise secondary containment. This involves containing each supply pipeline in a respective secondary containment pipeline that is optionally sealed at its ends to the fuel supply pipelines. The secondary containment pipeline prevents leaks from the fuel supply pipeline from being discharged into the environment, at the same time providing the facility to convey any leaked petrol to a remote sensing device, and allows the use of leak monitoring equipment.
Whilst every effort is made to avoid having joints in an underground supply pipeline, other than inside manhole chambers and sumps, these joints sometimes cannot be avoided. Such joints are conventionally made using special fittings and the connections are made using a chemical-based jointing compound or by electrofusion welding. Whilst the latter technique is preferred in many applications, it will be appreciated that any conventional or yet to be discovered methods for joining plastics components can be used to join the various parts of the coupling components and coupling assemblies together, including the use of bolt and gasket type methods. Whilst the techniques and components of the present invention will be described mainly in relation to electrofusion coupling components and electrofusion coupling assemblies, the present invention applies and works equally well with non-electrofusion coupling components, assemblies and techniques for joining the various plastics components together.
In conventional single containment plastic piping systems, successive lengths of plastic pipe are joined end to end using so-called electrofusion couplings or welding muffs, which typically comprise short plastic sleeves providing sockets at either end having internal diameters of a size to receive the ends of the respective pipes as a close sliding fit and incorporating electrical resistance heating windings. Thus two adjoining pipe lengths can be connected end to end by inserting the adjoining pipe ends into such an electrofusion coupler from opposite ends thereafter passing electric current through the heating windings in order to fuse the internal surfaces of the electrofusion coupling and the adjacent external surfaces of the inserted pipe ends, thereby welding the pipe ends to the electrofusion coupling to form a fluid tight joint.
Conventional electrofusion couplings require at least two electrical terminal connections on their outer surface. These electrical connectors, which are typically in the form of strong, metal terminal pins surrounded by a plastic shroud, are required in order to connect the connector fitting to an electrofusion welding machine. These electrical connectors therefore project significantly proud of the surface of the main body of the electrofusion coupling. Furthermore, the body of the electrofusion coupling itself has a finite thickness. The walls of these couplings have to be relatively thick, at least as thick as the walls of the pipe they are designed to join. It is important that these electrofusion couplings do not distort during the electrofusion process. It will also be understood that the electrofusion couplings fit over the outside of the pipe sections they are designed to join. As a consequence of the external location of the electrofusion coupling, the thickness of the body of the electrofusion coupling itself, and the projecting electrical connectors, the effective external diameter of the pipework in the region of the coupling is significantly greater than the external diameter of the pipe itself.
When these joins are made in a primary pipeline, this presents a problem regarding sizing the associated secondary pipe. One solution is to size the secondary pipe such that its internal diameter is large enough to accommodate the electrofusion fitting(s) joining the primary pipe. This solution provides for the minimum number of electrofusion joints required in the secondary pipework but is costly in materials in that a secondary pipe of a significantly larger diameter is used than would be necessary to enclose a primary pipe free from any joints.
A further solution is to use a secondary pipe that is a snug fit around the primary pipe and then use a series of expanders and reducers to increase the effective internal diameter of the secondary pipe in the region of any primary pipework joints. This solution has a number of inherent problems. For example, it will be appreciated that it is desirable to keep the number of such joints to a minimum. A minimum of four separate electrofusion joints are needed in this type of solution. This solution is also expensive in the number of separate components required. In addition, it is desirable if the various components of the secondary coupling are kept out of the way, whilst being in their correct relative positions, whilst the primary pipework is joined and tested. This can be difficult in on-site conditions.
In an attempt to reduce the number of components associated with a secondary containment joint and simplify the installation of the joint, the use of a coupler that is able to join secondary containment pipe sections together in a single welding step has been proposed in EP1,062,454 B1 and EP1,004,810 B1. Both documents disclose a joint for a secondary containment section. The joint comprises a single walled body comprising a seat on its internal surface as well as a channel passing there through. The arrangement of the seat on the inner surface of the body means that both the inner and outer pipes of the secondary containment pipe sections can be secured in position to the inner surfaces of the body by electrofusion welding. The presence of the channel within the body means that the continuity of the interstitial space between the separate secondary contained pipe sections is maintained along the length of the pipe as a whole and through the joints. Although pipe joints of this type facilitate the union of secondary containment pipe sections in a quick and simple manner, without the need for reducers, the single walled nature of the joint means that if any rupture or failure should occur within the walls of the joint, leakage of fluid contained within the pipe system will inevitably occur due to the single walled nature of the joint.
A further major disadvantage of this type of fitting is that they do not allow for the primary pipes to be joined and tested before the secondary pipe joints are made.
A further type of electrofusion fitting intended to accommodate both primary and secondary pipes is described in EP1,787,055 (Glynwed Pipe Systems Ltd). The complex shaped fittings described therein are both expensive to manufacture because of their intricate design and still require expansion/reduction fittings to complete the sealing assembly around the secondary pipework.
There is therefore a need for a simple, easy to manufacture coupling for use around a conventional primary pipe coupling, elbow or T.